The invention relates to a variable transmission gear with a transmission element such as a chain or belt, which transmission element is in engagement with a driving and a driven rosette, one rosette connected with a carrier from which or to which a torque is transmitted, whereby said rosette is formed by segments guided slidably with respect to the carrier.
At actually known embodiments of variable transmission gears the rosette consists of a number of parts which in case of a change of the transmission ratio are radially shifted with respect to or from the center of the rosette. Radial guiding means have to be provided for the shifting of individual parts of the rosette and also additional means are used, usually a spindle mechanism of an additional plate with a spiral groove, into which individual parts of the rosette are engaging. These arrangements have the drawback of an application of a large number of components of the transmission gear, whereby they are relatively expensive both in construction and in manufacture.
Another drawback of these variable transmission gears is the limiting of the extent of changes of the transmission ratio due to the limit of length of guidings of segments by a central supporting bolt.
It is thus impossible to obtain at these mentioned variable transmission gears the advantages of oval rosettes where in the course of a single revolution of the supporting shaft the length of the power arm on the rosette is cyclically changing, so that it is possible to synchronize the maximum power generated by the cyclist with the maximum transmission ratio in the course of the one revolution of the shaft as is the case at systems known by the name of "biopace".
At other known constructions of variable transmission gears the individual segments are arranged rotatably on a carrier. The rotational arrangement of segments is more demanding on an accurate guiding of segments and a lower stiffness is mostly caused thereby. Another drawback of rotational arrangements at a variable transmission gear is the circumstance that by this construction a constant prior required adjustment of the crank with respect to a chosen point on the rosette within the whole range of gear shifting is not achieved.